In recent years, the devlopment of plastic engineering materials has progressed to the point where, in many applications, a suitable plastic can be found to replace a metal structural component. Previous to this, plastic materials were generally not strong and versatile enough for such use. If the plastics were strong enough, they were brittle, and if developed to be strong and flexible, they were not durable.
With the development of improved plastic engineering materials, the trend in replacement and redesign using plastic components became noticeable especially in the automotive industry, and also in furniture design. The benefits to be derived from the use of plastic materials include reduced cost and manufacturing time, reduced weight and many other benefits.
Today, plastics such as polyamides, thermoplastic polyester and polycarbonates which can be impact modified, fiberglass reinforced and carbon fiber reinforced, have become suitable for many applications in which metal components were previously used. These applications include those requiring a tensile strength of over 24,100 lbs/sq-in. and an impact strength of over 3-4 lbs/in, and in which the components are required to perform under large amounts of stress for a long period of time.
In many prior art designs of modern office furniture, a gas spring is used in a support column to enable height adjustment of a seat or table surface, and these designs are disclosed in patents such as U.S. Pat. No. 4,113,220 to Collignon et al, U.S. Pat. No. 4,257,582 to Wirges, and U.S. Pat. No. 4,662,681 to Favaretto. Most of these designs use a metal stand pipe which is inserted into the chair base to provide a support column for the fluid spring, although the Favaretto patent discloses use of a plastic seat column which surrounds the metal stand pipe. The seat column is subjected to large amounts of stress, and this is problematic for a plastic seat column design.
The stand pipe is the structural component between the seat and the chair base which absorbs the stress placed on the chair, and has two major functions: 1) to guide and support the smooth, free and accurate movement of the fluid spring, and 2) to protect the fluid spring from breakage and distortion due to the weight and movement of the person occupying the chair, due to high stresses placed on the chair through the fluid spring. The standpipe must withstand the stress due to 400 lbs, at a height of approximately 3 feet.
The commonly used metal stand pipe is typically manufactured as a hollow steel cylinder weighing approximately 1.3 lbs, which is welded to a base at one end. At its other end there is fitted a molded plastic bushing which has a hole reamed in it, with the bushing riveted in place. The steel cylinder is then provided with a chrome-plated or painted finish.
The disadvantages of using a metal stand pipe include the relatively high price of manufacturing, including cutting, grinding and finishing, and the fact that the manufacturing and assembly steps require large amounts of time.
In addition, there is a problem with uniformity in manufacture, since each unit must be accurately processed through the same steps. The overall result is that out of every ten stand pipes, three or four have problems of alignment between the hole at the bottom of the cylinder and the hole in the plastic bushing fitted into the top end. To compensate for this, the hole in the stand pipe bottom is deliberately oversized, to provide the needed tolerance for the piston passing through the stand pipe. This sometimes causes noise or shaking of the fluid springs, detracting from the overall performance.
Therefore, it would be desirable to replace the metal stand pipe with a plastic one, to take advantage of the benefits afforded by use of high-strength plastic engineering materials.